SnCl2-Mediated gem-Difluoroallylation

ORGANIC LETTERS

Highly Diastereoselective Zn/ SnCl2-Mediated gem-Difluoroallylation of Chiral Hydrazones

2009 Vol. 11, No. 1 73-76

Xuyi Yue,† Xingang Zhang,† and Feng-Ling Qing*,†,‡ Key Laboratory of Organofluorine Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, 354 Fenglin Lu, Shanghai 200032, China, and College of Chemistry, Chemical Engineering and Biotechnology, Donghua UniVersity, 2999 North Renmin Lu, Shanghai 201620, China [email protected] Received October 13, 2008

ABSTRACT

gem-Difuoroallylic zinc reacts with chiral hydrazones 1 in the presence of SnCl2 to afford the versatile and chiral fluorinated building blocks, gem-difluorohomoallylic amines, in good yields and high diastereoselectivities.

Fluorinated compounds play a very important role in life sciences due to the introduction of one or a few fluorine atoms into an organic molecule showing profound changes in its chemical and biological nature.1 Among them, gem-difluorinated molecules constitute a distinct class of fluorinated compounds.2 Incorporation of a gem-difluoromethylene group (CF2) into an organic molecule can not only enhance its neighboring group stability or acidity but also act as an isopolar-isosteric substitute for oxygen.3 It has been used as one strategy for modification of biologically active compounds, †

Shanghai Institute of Organic Chemistry Donghua University (1) For recent reviews, see: (a) Smart, B. E. J. Fluorine Chem. 2001, 109, 3. (b) Maienfisch, P.; Hall, R. G. Chimia 2004, 58, 93. (c) Special issue on “Fluorine in the Life Sciences”, ChemBioChem 2004, 5, 557. (d) Purser, S.; Moore, P. R.; Swallow, S.; Gouverneur, V. Chem. Soc. ReV. 2008, 37, 320. (2) For reviews, see: (a) Tozer, M. J.; Herpin, T. F. Tetrahedron 1996, 52, 8619. (b) Reddy, V. P.; Perambuduru, M.; Alleti, R. AdV. Org. Synth. 2006, 2, 327. (3) Backburn, G. M.; England, D. A.; Kolkmann, F. J. Chem. Soc., Chem. Commun. 1981, 930. ‡

10.1021/ol802361p CCC: $40.75 Published on Web 12/08/2008

 2009 American Chemical Society

such as enzyme inhibitors,4 and anticancer agents (e.g., Gemcitabine5 and Vinflunine6 have been extensively used for treatment of lung, ovarian, renal, pancreatic, head and, neck cancers). Therefore, many efforts have been focused on the synthesis of gem-difluorinated compounds.2,7 The most commonly used methods thus far are direct difluorination of (4) (a) Furth, P. S.; Robinson, C. H. Biochemistry 1989, 28, 1254. (b) Akahoshi, F.; Ashimori, A.; Sakashita, H.; Yoshimura, T.; Eda, M.; Imada, T.; Nakajima, M.; Mitsutomi, N.; Kuwahara, S.; Ohtsuka, T.; Fukaya, C.; Miyazaki, M.; Nakamura, N. J. Med. Chem. 2001, 44, 1297. (c) Cox, R. J.; Hadfield, A. T.; Mayo-Martin, M. B. Chem. Commun. 2001, 18, 1710. (5) Hertel, L. W.; Kroin, J. S.; Misner, J. W.; Tustin, J. M. J. Org. Chem. 1988, 53, 2406. (6) Yun-San Yip, A.; Yuen-Yuen Ong, E.; Chow, L. W.-C Expert Opin. InVestig. Drugs 2008, 17, 583. (7) For recent selected examples, see: (a) Li, Y.; Hu, J. Angew. Chem., Int. Ed. 2005, 44, 5882. (b) Xu, B.; Hammond, G. B. Angew. Chem., Int. Ed. 2005, 44, 7404. (c) Xu, B.; Mashuta, M. S.; Hammond, G. B. Angew. Chem., Int. Ed. 2006, 45, 7265. (d) Li, G.; van der Donk, W. A. Org. Lett. 2007, 9, 41. (e) Ni, C.; Liu, J.; Zhang, L.; Hu, J. Angew. Chem., Int. Ed. 2007, 46, 786. (f) Li, Y.; Hu, J. Angew. Chem., Int. Ed. 2007, 46, 2489. (g) Codelli, J. A.; Baskin, J. M.; Agard, N. J.; Bertozzi, C. R. J. Am. Chem. Soc. 2008, 130, 11486. (h) Schuler, M.; Silva, F.; Bobbio, C.; Tessier, A.; Gouverneur, V. Angew. Chem., Int. Ed. 2008, 47, 7927.

carbonyl with DAST8 and the Reformatsky reaction using bromodifluoroacetate.9 However, to incorporate a gemdifluoromethylene group to an organic molecule in a chemo-, regio-, and stereo-manner is still a great challenge.10 Optically pure gem-difluorohomoallylic amines are very important compounds. They can not only be transformed into various new chiral fluorinated building blocks by functionization of alkene moiety, such as substituted alkenes, amino acids, δ-lactams, and pyrrolidines, but also be applied to biologically interesting compounds. In addition, allylic fluorides are potent active-site-directed irreversible inhibitors of isopentenyl-diphosphate isomerase.11 Surprisingly, only two examples of preparation of gem-difluorohomoallylic amines have been reported,12 despite that impressive progress has been made on the development of asymmetric synthesis of nonfluorinated homoallylic amines.13,14 One of these two methods is through direct difluorination of allylic ketone with DAST. But the yield was poor, intricate transformations were required, and many functional groups were incompatible under the harsh reaction conditions.12a The other one was developed by our group. However, it required the conversion of a chiral gem-difluorohomoallylic alcohol into an amine.12b,c On the basis of a great demand for difluorinated structures as well as the importance of optically pure gemdifluorohomoallylic amines in biological chemistry and natural product modification, a direct and efficient method would thus facilitate their accessibility. Herein, we disclose the first successful, efficient, highly stereoselective synthesis of gem-difluorohomoallylic amines through the coupling of the gem-difluoroallylic zinc with chiral hydrazones in the presence of SnCl2 in good yields with high diastereoselectivities (in many of the cases, only a single diastereoisomer was obtained). Enantioselectivities up to >99% ee could be achieved after removal of the chiral auxiliary. (8) (a) Middleton, W. J. J. Org. Chem. 1975, 40, 574. (b) Hudlicky, M. Org. React. 1988, 35, 513. (c) Takeo, T. J. Synth. Org. Chem. Jpn. 1990, 48, 1048. (9) For recent examples, see: (a) Konas, D. W.; Pankuch, J. J.; Coward, J. K. Synthesis 2002, 17, 2616. (b) Ocampo, R.; Dolbier, W. R., Jr.; Zuluaga, F. J. Org. Chem. 2002, 67, 72. (c) Cozzi, P. G. Angew. Chem., Int. Ed. 2007, 46, 2568. (10) Ma, J. -A.; Cahard, D. Chem. ReV. 2004, 104, 6119. (11) Muehlbacher, M.; Poulter, C. D. Biochemistry 1988, 27, 7315. (12) (a) Ohba, T.; Ikeda, E.; Takei, H. Bioorg. Med. Chem. Lett. 1996, 6, 1875. (b) Wu, Y.-Y.; Zhang, X.; Meng, W. -D.; Qing, F. -L. Org. Lett. 2004, 6, 3941. (c) Yue, X.; Wu, Y. -Y.; Qing, F. -L. Tetrahedron 2007, 63, 1560. (13) For examples of enantioselective metal-catalyzed allylation of imines, see: (a) Fernandes, R. A.; Stimac, A.; Yamamoto, Y. J. Am. Chem. Soc. 2003, 125, 14133. (b) Taggi, A. E.; Hafez, A. M.; Lectka, T. Acc. Chem. Res. 2003, 36, 10. (c) Hamada, T.; Manabe, K.; Kobayashi, S. Angew. Chem., Int. Ed. 2003, 42, 3927. (d) Wada, R.; Shibuguchi, T.; Makino, S.; Oisaki, K.; Kanai, M.; Shibasaki, M. J. Am. Chem. Soc. 2006, 128, 7687. (14) For examples of asymmetric allylation of hydrazones, see: (a) Kobayashi, S.; Ogawa, C.; Konishi, H.; Sugiura, M. J. Am. Chem. Soc. 2003, 125, 6610. (b) Berger, R.; Rabbat, P. M. A.; Leighton, J. L. J. Am. Chem. Soc. 2003, 125, 9596. (c) Ogawa, C.; Sugiura, M.; Kobayashi, S. Angew. Chem., Int. Ed. 2004, 43, 6491. (d) Berger, R.; Duff, K.; Leighton, J. L. J. Am. Chem. Soc. 2004, 126, 5686. (e) Tan, K. L.; Jacobsen, E. N. Angew. Chem., Int. Ed. 2007, 46, 1315. (f) Kargbo, R.; Takahashi, Y.; Bhor, S.; Cook, G. R.; Lloyd-Jones, G. C.; Shepperson, I. R. J. Am. Chem. Soc. 2007, 129, 3846. (15) Cook, G. R.; Maity, B. C.; Kargbo, R. Org. Lett. 2004, 6, 1741. (16) (a) Shibasaki, M.; Matsunaga, S. Chem. Soc. ReV. 2006, 35, 269. (b) Ikariya, T.; Blacker, A. J. Acc. Chem. Res. 2007, 40, 1300. (c) Paull, D. H.; Abraham, C. J.; Scerba, M. T.; Alden-Danforth, E.; Lectka, T. Acc. Chem. Res. 2008, 41, 655. 74

Inspired by Cook’s indium-mediated diastereoselective allylation of chiral hydrazones,15 initially we started the selective synthesis of chiral gem-difluorohomoallylic amine by coupling of gem-difluoroallylic indium with chiral hydrazone 1a in which the valinol-derived oxazolidinone was employed as a chiral auxiliary. To our surprise, the reaction between 1a and the 3-bromo-3,3-difluoropropene 2 in the presence of indium powder did not afford any product, which is in strong contrast to Cook’s nonfluorinated results15 (Scheme 1). Investigation of different solvent systems

Scheme 1. Metal-Mediated Non- and Difluorianted Allylation of Chiral Valinol Hydrazone 1a

(DMSO, THF, or THF/H2O), metals (In, Al/Sn), or elevating the reaction temperature did not give the desired product either. We surmised that there might be two reasons for this negative result. First, the strong electron-withdrawing effect of fluorine dramatically reduces the nucleophilicity of gemdifluoroallylic metal species toward electrophiles. Second, compared to imines, the oxazolidinone-substituted hydrazones are weak electrophiles. Thus, we assumed that using a Lewis acid to activate the chiral hydrazones or a bifunctional catalyst16 to activate both a nucleophile and an electrophile would promote this coupling reaction. On the basis of these considerations, we then examined the indiummediated gem-difluoroallylation of 1a in DMF in the presence of In(OTf)3 or InCl3 (Table 1, entries 1 and 2). However, no desired product was formed. Switching solvent to THF did not afford 3a (Table 1, entry 3). Since the gemdifluoroallylic zinc generated in situ is more reactive than its corresponding indium species, the zinc-mediated addition of 2 to 1a in the presence of different Lewis acids was screened. We discovered that when the reaction was performed with zinc powder in THF at room temperature in the presence of TMSCl trace 3a was provided (Table 1, entry 5). Encouraged by this result, other Lewis acids BiCl3, CeCl3, and SnCl2 were tested, where SnCl2 was supposed to be a bifunctional catalyst for the addition of allylic zinc to imines, Sn (Lewis acid) toward imines, and Cl (Lewis base) toward Zn of allylic zinc halide17 (Table 1, entries 6-8, Figure 1). To our delight, when 2.0 equiv of SnCl2 was investigated, 41% yield of desired product 3a was afforded. More gratifyingly, 19F NMR of the crude product showed that only Org. Lett., Vol. 11, No. 1, 2009

Table 1. Metal-Mediated Addition of 3-Bromo-3,3-difluoropropene 2 to Chiral Hydrazone 1a in the Presence of Lewis Acid

entry

metal

solvent

Lewis acid

yield [%][a]

1 2 3 4 5 6 7 8 9

In In In In Zn Zn Zn Zn Zn

DMF DMF THF THF THF THF THF THF THF

In(OTf)3 InCl3 In(OTf)3 SnCl2 TMSCl BiCl3 CeCl3 SnCl2 (2.0equiv) SnCl2 (0.4equiv)

NR[b] NR[b] NR[b] NR[b] trace[c] trace[c] trace[c] 41% 92%

Table 2. Zinc-Mediated Addition of 3-Bromo-3,3-difluoropropene 2 to Chiral Hydrazones 1 in the Presence of SnCl2[a]

[a] Determined by gas chromatography. [b] No reaction. [c] Determined by 19F NMR.

Figure 1. Mechanistic proposal on stereocontrol.

a single diastereoisomer of 3a was formed. Further optimizing reaction conditions, we found that using 0.4 equiv of SnCl2 and 5 equiv of zinc powder the reaction could be conducted in 92% yield without erosion of the diastereoselectivity (Table 1, entry 9). The amount of SnCl2 has a notable effect on the reaction. Overloading or reducing the amount of SnCl2 would lead to decreased yield, but high stereoselectivities remained (for details, see Supporting Information Table 1, S10). On the contrary, when SnCl2 was used for the indium-mediated addition of 2 to 1a, no desired product was detected (Table 1, entry 4). Under the optimized condition, the scope of gem-difluorohomoallylation of chiral hydrazones 1 with gem-difluoroallylic zinc in the presence of SnCl2 was investigated (Table 2), and the hydrazones included those bearing electron-withdrawing and electron-donating substituents on the aryl ring and aliphatic and functional hydrazones. High diastereoselectivities (de 12/1 to single, determined by 19F NMR) and yields ranging from 42% to 86% were observed for all the hydrazones tested. For all the aryl substituted hydrazones, the yields depend on the electronic (17) (a) Olah, G. A.; Kobayashi, S.; Tashiro, M. J. Am. Chem. Soc. 1972, 94, 7448. (b) Mine, N.; Fujiwara, Y.; Taniguchi, H Chem. Lett. 1986, 357. (c) Amer, I.; Alper, H. J. Am. Chem. Soc. 1989, 111, 927. (d) Grushin, V. V.; Alper, H. Organometallics 1993, 12, 3846. Org. Lett., Vol. 11, No. 1, 2009

[a] Reaction conditions: 1 (1 equiv), 2 (7 equiv), Zn (5 equiv), SnCl2 (0.25 M in THF, 0.4 equiv), room temperature, 0.1 M in THF. [b] Yield of isolated product. [c] Yield based on the conversion. [d] 300 mg of 4 Å molecular sieves/mmol substrate was added. [e] Determined by 19F NMR before column chromatography. [f] 19F NMR showed only a single diastereoisomer was detected.

features of the substituents, and 4 Å molecular sieves were required to facilitate the reaction (Table 2, entries 1-9). Generally, moderate to good yields could be achieved for the aromatic hydrazones, despite that the starting materials could not be consumed completely. Electron-rich aryl hydrazones provided moderate yields with high de values (Table 2, entries 3 and 4), while electron-deficient aryl substituents afforded good yields with good to excellent diastereoselectivities (Table 2, entries 5-9). Hydrazones bearing a naphthyl, furyl, or thienyl group also furnished 3j-3l in good yields with high diastereoselectivity (Table 2, entries 10-12), but a slightly decreased selectivity for substrate 1k (Table 2, entry 11). Notably, all alkylsubstituted hydrazones underwent complete reaction in good yields, and only a single diastereoisomer of desired product was detected by 19F NMR (Table 2, entries 14-16). Functional hydrazones 1q-1s also tolerated the reaction with de 12/1 to single (Table 2, entries 17-19), and only the 1,2-addition product was obtained for cinnamyl-substituted hydrazone 1m 75

(Table 2, entry 13). It should be mentioned that the optically pure 3r resulting from 1r is a very important building block to access many bioactive β-fluorinated amino acids18 via the transformation of an alkene moiety (Table 2, entry 18). The absolute configurations of the hydrazines were assigned by X-ray crystallographic analysis of optically pure 3e (for details see Supporting Information S27), which indicated that the selectivity of the addition of hydrazones with gem-difluoroallylic zinc should be induced from the less hindered Si face of the hydrazones to generate the R configuration. The greater reactivity of the substrates and high stereoselectivity of the reaction probably could be explained by the coordination of SnCl2 with substrates in a highly restricted chair transition state in which Sn coordinates C)N of hydrazone, and carbonyl group of oxazolidinone to activate the hydrazone and Cl of SnCl2 chelates with Zn to activate the gem-difluoroallylic zinc, which allows higher reactivity of both the electrophile and nucleophile and induces the gem-difluoroallylic zinc to attack the steric less hindered Si face of hydrazone in a high stereoselective manner (Figure 1). The optical pure primary gem-difluorohomoallylic amines could be efficiently obtained by Friestad’s procedure for cleavage of the N-N bond of hydrazides.19 As shown in Scheme 2, compound 3a was trifluoroacylated with trifluoroacetic anhydride (TFAA), and the resulting N-trifluoroacylated hydrazine 4a was exposed to SmI220 to afford gem-difluorohomoallylic amide 5 in 55% yield with up to >99% ee. (18) (a) Kukhar, V. P.; Soloshonok, V. A. Fluorine Containing Amino Acids: Synthesis and Properties; Wiley: New York, 1995. For recent reviews, see: (b) Sutherland, A.; Willis, C. L. Nat. Prod. Rep. 2000, 17, 621. (c) Qiu, X. -L.; Meng, W. -D.; Qing, F. -L. Tetrahedron 2004, 60, 6711. (19) Ding, H.; Friestad, G. K. Org. Lett. 2004, 6, 637. (20) Souppe, J.; Danon, L.; Namy, J. L.; Kagen, H. B. J. Organomet. Chem. 1983, 250, 227.

76

Scheme 2. Cleavage of the Hydrazine N-N Bond

In conclusion, we have developed a highly diastereoselective Zn/SnCl2 mediated gem-difluoroallylation of chiral hydrazones. The bifunctional catalyst SnCl2 plays an important role in the reaction. The reaction scope can be extended to a series of aromatic, heteroaromatic, aliphatic, and functional hydrazines in good yields with high diastereoselectivities. In many of the cases (both aromatic and aliphatic hydrazones), only a single diastereoisomer could be detected by 19F NMR. To the best of our knowledge, this represents the first example of highly asymmetric gemdifluoroallylation of stable, isolable imine equivalents. The potential synthetic utility of this highly stereoselective gemdifluoroallylation reaction is in progress. Acknowledgment. This paper is dedicated to Prof. QinYun Chen on the occasion of his 80th birthday. The National Natural Science Foundation of China and Shanghai Municipal Scientific Committee are greatly acknowledged for funding this work. Supporting Information Available: Detailed experimental procedures and analytical data for all new compounds and crystallographic data for compound 3e. This material is available free of charge via the Internet at http://pubs.acs.org. OL802361P

Org. Lett., Vol. 11, No. 1, 2009